Separators

Battery separators are used to connect a house battery to a starting battery, so that each can be charged from the others’ charging source. The separator only joins the batteries when one is fully charged.

There are two types of separator: one way (or unidirectional), and two way (or bidirectional).

Wiring either type is very simple:

heavy gauge wire from each battery to the appropriate stud on the separator

light-gauge wire (14AWG is recommended) from the earth tab of the separator to chassis ground

Separators vs isolators

I don’t talk much about isolators in these pages. Isolators use a diode to isolate one battery from the other; voltage traveling across this diode experiences a loss of about 0.6 volts. This can result in a charge voltage too low to be effective. If the isolator fails, the batteries are left in a separated state.

Separators use a solenoid switch, with no voltage drop. If the switch fails, the batteries are left in a connected state.

One way separators

These allow charging of both batteries from a single side of the separator. Usually, this is set up to allow the house battery to be charged by the alternator, once the starting battery is fully charged.

Two way separators

These allow charging of both batteries from a charging source on either side of the separator. In this way, the alternator will charge the starting battery always, and the house battery once the starting battery is charged; also, a charging source on the house battery side (such as a shore power charger or solar panel) will charge the starting battery when the house battery is charged.